1. Technical Field
This invention relates to gas turbine engines having variable stage vanes in general, and to apparatus for actuating variable stage vanes in particular.
2. Background Information
Vane assemblies increase efficiency and performance within gas turbine engines by directing air at an optimum flow path for downstream components. The flow path of air exiting a vane is influenced by the orientation, or the "angle of attack", of the vane. In some sections of the engine, the optimum angle of attack varies with the thrust setting of the engine and "where" the engine is within its flight envelope. Hence, stationary vanes only provide an optimum air flow path for a portion of the performance envelope of the engine. Variable stage vanes, on the other hand, may be manipulated to change the angle of attack and consequently can provide an optimum air flow path for a variety operating conditions.
Variable vane assemblies typically include a plurality of vanes circumferentially distributed and pivotly disposed between an inner vane support and an outer casing. Each vane typically includes a post extending up through the outer casing and a pivot arm fixed to the post on the opposite side of the outer casing. The fixed attachment between each vane and pivot arm causes the pivot arms and the vanes to pivot together about the same axis. All of the pivot arms are pivotly attached to a synchronizing ring disposed between, and concentric with, the outer casing and the nacelle (or engine bay depending upon the application). An actuator provides the means for driving the synchronizing ring along the circumference of the outer casing.
When a change in operating conditions makes it advantageous to change the vane angle of attack, the actuator is directed to circumferentially rotate the synchronizing ring to a new circumferential position associated with a particular vane angle of attack. The pivot arms, and the vanes fixed to the pivot arms, rotate with the synchronizing ring. Under ideal circumstances, the synchronizing ring is concentric with the outer casing and readily rotated between positions. Under more common circumstances, however, air flow forces acting against the vanes force the synchronizing ring out of round, and into contact with the outer casing. Contact between the synchronizing ring and outer casing inhibits motion and can prevent proper positioning of the ring.
The point at which the pivot arm acts on the synchronizing ring also affects the roundness of the ring. Pivot arms attached to the inner or outer radial surface of the synchronizing ring produce moments which, if of sufficient magnitude, can increase deflection of the ring and add to any out of round condition that may exist. Moments acting on the ring can also introduce additional undesirable stresses within the ring.
In short, what is needed is an apparatus for actuating variable vanes that facilitates actuation by maintaining concentricity with the outer casing and minimizing stress within the synchronizing ring.